Heat-developable photosensitive materials which can produce photographic images using a dry heat processing method are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075. These Patents disclose photothermographic elements comprising an organic silver salt, a catalytic amount of a photocatalyst, e.g. silver halide, and a reducing agent. The photothermographic materials are stable at ambient temperatures but when heated to a temperature of above 80.degree. C., preferably 100.degree. C. or higher, after imagewise exposure, produce silver through a redox reaction between the organic silver salt (acting as an oxidising agent) and the reducing agent. This redox reaction is accelerated by the catalytic action of the exposure generated silver catalyst. The silver which is produced by reduction of the organic silver salt in the exposed areas provides a black image to produce a contrast with respect to the unexposed areas. This results in the formation of an image.
In practice, it is essential to include an effective antifoggant in such photothermographic materials since, without an antifoggant, some generation of silver in the unexposed areas takes place upon thermal development, resulting in a poor differential between the image and background fog. In the past, the most effective antifoggant has been mercuric ion. The use of mercury compounds as antifoggants in photothermographic materials is disclosed in, for example, U.S. Pat. No. 3,589,903.
However, mercury compounds are environmentally undesirable and due to an increasing desire to remove even trace amounts of possible pollutants from commercial articles there is a demand to find equally effective but less hazardous antifoggants.
Various compounds have been suggested for use as antifoggants in place of mercury compounds in photothermographic elements.
U.S. Pat. No. 4,546,075 discloses, as antifoggants in place of mercury compounds, the use of compounds of the general formula: ##STR2## in which: R represents a halogen atom, and
R.sup.1 represents hydrogen, alkyl, aryl, aralkyl, acyl, carbamoyl, alkylsulfonyl, arylsulfonyl or a heterocycle, and the use of compounds of the general formula: ##STR3## in which: n is an integer of 1 to 4, PA0 X represents S, O, NR.sub.2, PA0 R represents a halogen atom, and PA0 R.sup.1 represents alkyl, aryl or acyl groups. PA0 X represents halogen, preferably Br, and PA0 R.sup.1 and R.sup.2 are optionally substituted acyl, oxycarbonyl, oxysulfonyl, alkylsulfonyl, arylsulfonyl, aralkylsulfonyl, carboxy, sulfo or sulfamoyl. PA0 R represents hydrogen or alkyl, aryl, aralkyl, alkenyl groups or a heterocyclic residue, each of which may be substituted. PA0 X.sup.3 represents a halogen atom such as bromine or chlorine, preferably bromine, or an electron withdrawing substituent, e.g. acyl, oxycarbonyl, oxysulfonyl, and PA0 Z represents the necessary atoms to complete a ring system which may comprise a single ring or a fused ring system which rings may bear substituents.
Japanese Patent Publication No. 59/57234 discloses, as antifoggants in place of mercury compounds in dry silver systems, the use of compounds of the formula: EQU R.sup.1 --CX.sub.2 --R.sup.2
in which:
U.S. Pat. No. 4,452,885 discloses, as antifoggants in place of mercury compounds, the use of compounds of the general formula: ##STR4## in which: X represents a halogen atom, and
An alternative group of compounds has now been found which are effective antifoggants in photothermographic elements and provide certain advantages over the use of both mercury antifoggants and the organic antifoggants of the prior art.